Explicit context information for unit conversion

[huard]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes The "hydro" context is always on. #1208
• Tests for the changes have been added (for bug fixes / features)
  • (If applicable) Documentation has been added / updated (for bug fixes / features)
• HISTORY.rst has been updated (with summary of main changes)
  • Link to issue (:issue:number) and pull request (:pull:number) has been added

What kind of change does this PR introduce?

• Disables hydro context
• Enable hydro context for specific operations
• Add infer_context function to encapsulate the logic determining context when not explicit (e.g. generic indices, datachecks, etc.)
• Adjust tests

Does this PR introduce a breaking change?

Things that worked out of the box by assuming the hydro context applied to every xclim operation will break. A bit more care needs to be taken when writing indices, especially generic ones, to handle unit conversion correctly.

Other information:

[aulemahal]

I find it non-ideal that the solution is as verbose as it is, but that's the current state of xclim's context management. And at least it is explicit.

I also added some conditions for infer_context, as well as a test for it!

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[aulemahal]

@huard I added the option to pass convert_units_to(src, tgt, context='infer') which will call infer_context on the standard names of either src or tgt. This allowed me to fix the remaining issues in sdba where we are dealing with dataarrays of an unknown unit. The default is still None.

[aulemahal]

@Zeitsperre It hung here too, so I simply commented it out.

[Zeitsperre]

This is really mysterious... I have a theory on what it could be but will explore more.